There is a continued need for improving the emissions quality of internal combustion engines. At the same time, there is pressure to minimize engine crank times and time from key-on to drive-away, while maintaining maximum fuel economy. Those pressures apply to engines fueled with alternative fuels such as ethanol as well as to those fueled with gasoline.
During cold temperature engine start, the conventional spark ignition internal combustion engine is characterized by high hydrocarbon emissions and poor fuel ignition and combustibility. Unless the engine is already at a high temperature after stop and hot-soak, the crank time may be excessive, or the engine may not start at all. At higher speeds and loads, the operating temperature increases and fuel atomization and mixing improve.
During an actual engine cold start, the enrichment necessary to accomplish the start leaves an off-stoichiometric fueling that materializes as high tail-pipe hydrocarbon emissions. The worst emissions are during the first few minutes of engine operation, after which the catalyst and engine approach operating temperature. Regarding ethanol fueled vehicles, as the ethanol percentage fraction of the fuel increases to 100%, the ability to cold start becomes increasingly diminished, leading some manufacturers to include a dual fuel system in which engine start is fueled with conventional gasoline and engine running is fueled with the ethanol grade. Such systems are expensive and redundant.
Another solution to cold start emissions and starting difficulty at low temperature is to pre-heat the fuel to a temperature where the fuel vaporizes quickly, or vaporizes immediately (“flash boils”), when released to manifold or atmospheric pressure. Pre-heating the fuel replicates a hot engine as far as fuel state is considered.
A number of pre-heating methods have been proposed, most of which involve preheating in a fuel injector. Fuel injectors are widely used for metering fuel into the intake manifold or cylinders of automotive engines. Fuel injectors typically comprise a housing containing a volume of pressurized fuel, a fuel inlet portion, a nozzle portion containing a needle valve, and an electromechanical actuator such as an electromagnetic solenoid, a piezoelectric actuator or another mechanism for actuating the needle valve. When the needle valve is actuated, the pressurized fuel sprays out through an orifice in the valve seat and into the engine.
One technique that has been used in preheating fuel is to provide a positive temperature coefficient ceramic heater designed into a fuel injector to heat the fuel surrounding the heater. An exemplary fuel injector having a ceramic heater is disclosed in U.S. Pat. No. 6,102,303. Another technique is the use of a resistively heated capillary tube within which fuel is passed to heat the fuel to vapor. An exemplary aerosol generator including a heated capillary tube is disclosed in U.S. Pat. No. 6,681,769. Both those solutions require electrical connections penetrating through the injector wall into the fuel passage, leading to an increased risk of fuel leakage. Those techniques further require separate conductors for providing power to the injector heater, complicating wiring harnesses and connectors.
Another method for pre-heating fuel is to inductively couple energy into the injector with a time-varying magnetic field. That can be done while maintaining a hermetically sealed fuel passage, as no electrical penetration is necessary. The energy is converted to heat inside a component suitable in geometry and material to be heated by the hysteretic and eddy-current losses that are induced by the time-varying magnetic field.
The inductive fuel heater is useful not only in solving the above-described problems associated with gasoline systems, but is also in pre-heating ethanol grade fuels to accomplish successful starting without a redundant gasoline fuel system.
Because the induction heating technique uses a time-varying magnetic field, the system must include electronics for providing an appropriate high frequency alternating current to an induction coil in the fuel injector.
Conventional induction heating is accomplished with hard-switching of power, or switching when both voltage and current are non-zero in the switching device. Typically, switching is done at a frequency near the natural resonant frequency of a resonator, or tank circuit. The resonator includes an inductor and capacitor that are selected and optimized to resonate at a frequency suitable to maximize energy coupling into the heated component.
The natural resonant frequency of a tank circuit is fr=1/(2π√{square root over (LC)}), where L is the circuit inductance and C is the circuit capacitance. The peak voltage at resonance is limited by the energy losses of the inductor and capacitor, or decreased quality factor, Q, of the circuit. Hard-switching can be accomplished with what are called half-bridge or full-bridge circuits, comprising of a pair or two pairs of semiconductor switches, respectively. The switches may be any number of semiconductor types, such as a thyristor, triac, PNP or NPN transistor, Darlington transistor, FET (Field Effect Transistor), MOSFET (Metal Oxide Semiconductor FET), IGBT (Insulated Gate Bipolar Transistor), or vacuum and gas tube types, such as krytron, thyratron, ignitron, tetrodes, etc. Hard-switching of power results in the negative consequences of switching noise, and high amplitude current pulses at resonant frequency from the voltage supply, or harmonics thereof. Also, hard switching dissipates power during the linear turn-on and turn-off period when the switching device is neither fully conducting nor fully insulating. The higher the frequency of a hard-switched circuit, the greater the switching losses.
In an engine environment, fuel injectors are coupled to the electronic controllers through a system of wiring harnesses and connectors. Heated fuel injectors have required additional conductors for driving the heating elements in the injectors, Those additional conductors have complicated the connectors and harnesses, and have increased expense and potential failure points in the wiring system.
There is therefore presently a need to provide a fuel injector heater circuit and method of driving a heated fuel injector wherein switching is done at the lowest possible interrupted power. There is furthermore a need to reduce the number of conductors used for each fuel injector. To the inventor's knowledge, no such controller or method is currently available.